This invention relates to crimping of composite electrical insulators for high, medium, or low voltage use.
A composite insulator comprises a structurally strong core or rod typically made of fibreglass, a series of electrically insulating sheds, and two metal end fittings crimped onto the exposed ends of the electrical insulator. The insulator assembly, and therefore the crimped joint must be able to withstand tensile forces as per particular specification (SML=Specified Mechanical Load) of the insulator.
A major failure mode of composite insulators is cracking of the fibre glass rod inside the metal end fitting during the crimping process. In this process, a hydraulic press is used to drive the dieset in the radial direction towards the rod. During the forward stroke, the dies crimp the circumference of the metal end fitting. This crimping action compresses the steel onto the fibre glass rod while permanently deforming the steel. Due to the specific material properties of fibre glass, such a rod has a great structural strength in its longitudinal direction but a limited structural strength in its radial direction.
Cracking during crimping occurs when the compressive stresses induced in the fibre glass rod due to over-crimping exceed the compressive strength of the rod in the transverse direction. Also, stress concentrations can also be induced due to surface roughness in the drilled bore in the steel end fitting. These stress concentrations can cause rod failures during crimping, resulting in a weak mechanical coupling between the rod and the end fittings.
The traditional method of crack detection throughout the industry is acoustic monitoring, that is using suitable acoustic monitors and amplifiers to detect the noise of cracking as it occurs during the crimping operation. It has shown, however, that the acoustic monitoring method is difficult to employ. In practice, therefore, the monitoring is often limited to occasional samples instead of entire production runs, resulting in some defective joints not being detected.
It is therefore an object of the present invention to provide a method of monitoring the crimping of metal end fittings onto a rod which provides greater reliability.
It is another object of the present invention to provide a method of monitoring the crimping of metal end fittings onto a rod which is economical and easy to employ.
It is still another object of the present invention to provide a crimping device capable of monitoring the crimping of metal end fittings onto a rod, such as a fibreglass rod.
Accordingly, a method of monitoring the crimping of metal end fittings onto an electrically insulating core rod of an electrical insulator using a crimping apparatus having crimping jaws is in accordance with the present invention characterised by
measuring the force and/or pressure applied to the end fittings by the crimping jaws during the crimping,
measuring the distance travelled by the jaws during the crimping,
detecting a non-increasing force and/or pressure with an increasing distance.
The present invention thus provides a novel method to detect rod failure by cracking or matrix failure during the crimping operation which uses force and/or pressure transducers to monitor and predict rod cracking. By using force and/or pressure transducers, a direct indication of the stresses in the fibre glass rod are obtained, in contrast to the indirect indication provided by acoustic monitoring. Also, the monitoring is carried out as a continuous process during the crimping operation and can be employed during an entire production run, thus offering greater reliability. Another advantage is the possibility to immediately discard the insulator when severe cracks are detected, thus saving additional process steps.
The use of transducers for monitoring crimping processes is known as such for crimping electrical connectors onto wires. European patent application EP 0,460,441, for example, discloses a method for determining the quality of an electrical connection when crimping an electical connector onto a metal wire. The quality of the electrical connection is monitored by collecting force and displacement data and comparing those data with standard data. There is no crimping onto a rod having a relatively fragile structure, such as a fibreglass rod. Also, the crimping process monitored is intended to provide a good electrical connection, whereas the quality of the mechanical connection and the resistance to tensile forces is only of secundary importance.
European patent application EP 0,397,434 also discloses a method for monitoring the crimping of electrical connectors onto metal wires and therefore addresses different problems than the present invention. A similar method of monitoring the crimping onto wire is disclosed in U.S. Pat. No. 5,168,736. None of these documents address the problems associated with crimping end fittings onto the fibre glass rod of an electrical insulator.
In the method of the present invention, the ratio of the force applied and the distance travelled and/or the ratio of the pressure applied and the distance travelled may be calculated and a change in any such ratio may be used to detect a substantially non-increasing force or pressure with an increasing distance. Alternatively, or additionally, the force applied and the distance travelled and/or the pressure applied and the distance travelled may be displayed to enable a visual detection of a non-increasing force and/or pressure applied with an increasing distance travelled.
Although the invention is explained by way of an embodiment in which the distance travelled is used to monitor the crimping process, the time elapsed during the crimping process may measured and used instead of or in addition to the distance travelled. When using the time elapsed as a variable it is preferred to detect a decrease in the force or pressure applied within a certain time period.
As explained above, the present invention provides a new and advantageous quality control method that can be used to detect failure of the fibre glass rod during the crimping process. Incorporating this technology into crimping machines will lead to improved quality assurance on the mechanical properties of the insulator. Accordingly, the present invention also provides a crimping apparatus having crimping jaws for crimping metal end fittings onto an electrically insulating core rod of an electrical insulator, which crimping apparatus is characterized by force and/or pressure transducers associated with the jaws so as to be capable of monitoring the progress of the crimping operation to detect over-crimping by measuring the force and/or pressure applied by the jaws to the end fittings being crimped and the distance travelled by the jaws.
Advantageously, the transducers are accomodated in crimping dies mounted on the jaws. This requires a modification of the dies only, not of the jaws. In a preferred embodiment, the dies consist of fixed master dies and interchangeable crimping dies, the transducers being accomodated in the master dies. This ensures that the transducers are present in the crimping device irrespective of the particular crimping dies used. Also, only a single transducer or set of transducers is necessary in this embodiment, as there is no need to provide the individual interchangeable crimping dies with transducers.